Magnetic resonance imaging device and gradient magnetic field coil used for it

ABSTRACT

To reduce vibration and noise generated in supplying pulsatile current to the gradient magnetic field coil for driving it in the MRI apparatus, the present invention discloses an active shielded gradient magnetic field coil, in which the gradient magnetic field coil is supported by and fixed to the static magnetic field generating magnet. The supporter for supporting this gradient magnetic field coil in non-contact does not prevent the interventional procedure performed by a doctor. Further, eddy current generated on the surface of the static magnetic field generating magnet near the periphery of the gradient magnetic field coil when supplying the pulsatile current to the gradient magnetic field coil can be reduced.

FIELD OF THE INVENTION

[0001] The present invention relates to a magnetic resonance imaging(hereinafter referred to as MRI) apparatus and a gradient magnetic fieldcoil used in said MRI apparatus. More particularly, it relates to an MRIapparatus employing an open magnet which does not give oppressivefeeling to an object to be examined, which provides the objectcomfortable conditions for examination by reducing noise, and to agradient magnetic field coil constructed so as to reduce eddy currentgenerated on the surface of a vessel of a magnet for generating a staticmagnetic field when current is applied to the gradient magnetic fieldcoil to perform imaging of the object.

BACKGROUND OF THE INVENTION

[0002] In an MRI apparatus, there has been conventionally employed amagnet for generating a static magnetic field comprising a narrowtubular solenoid coil, which can efficiently generate a uniform staticmagnetic field. However, the MRI apparatus of this type puts an objectto be examined in a space like a tunnel, imparting an oppressive feelingto the object. It is also reported that it gives fear to aclaustrophobic person and a child. Accordingly, an open MRI apparatusemploying a flat type gradient magnetic field coil, in which one sideface of the magnetic device is closed off and the front face of themagnet is opened up to widen the space for the object to enter, hasrecently seen increasing use.

[0003] Since such an open MRI apparatus using the open magnet has an airspace to enable interventional procedure in which a doctor performstreatment while observing an image, advanced medical institutions hasbeen employing it in recent years. In said MR-interventional procedure,it is demanded that the operator can check the MR image of the procedurebeing carried out in real time. To improve the image quality and thefunctions of the real-time imaging (high-speed imaging), the MRIapparatus requires a gradient magnetic field coil that operates at highspeed and a driving power for this gradient magnetic field coil, an RFcoil for detecting NMR signals with high sensitivity, and a magnethaving high static magnetic field strength.

[0004] A magnet having high static magnetic field strength required forthe open MRI apparatus is proposed in Japanese Patent Laid-openPublication JP-A-10-179546, JP-A-11-155831, and JP-A-11-197132.

[0005] As for the gradient magnetic field coil, on the other hand, aswitching power having large capacity has been developed, and thus thegradient magnetic field coil can be driven at high-speed. However,electromagnetic force works on the gradient magnetic field coil whencurrent for driving the gradient magnetic field is applied in pulseform, and mechanical distortion and vibration occur, this vibrationbeing transmitted to the magnet for generating the static magneticfield. Thus, there occur some problems that the uniformity of themagnetic field in the measurement space is deteriorated and thatacoustic noise due to the vibration of the gradient magnetic field coilis raised.

[0006] Those problems are far more serious in the MRI apparatus havingan open-gantry structure. That is, the open MRI apparatus is more liableto be affected by the vibration than the apparatus employing a tunneltype magnet. The vibration transmitted from the gradient magnetic fieldcoil to the magnet deteriorates the stability of the static magneticfield generated by said magnet, and thus the image quality isdeteriorated.

[0007] Further, as described above, both noise and vibration increase inthe high-speed imaging in real time since the gradient magnetic fieldcoil is subject to rapid switching-drive. For example, when performingan imaging procedure such as EPI method in which the gradient magneticfield coil is subject to rapid switching-drive, operating noise of thegradient magnetic field can be over 100 dB.

[0008] Conventionally, the techniques described below have been proposedfor solving said problems concerning the noise and vibration of thegradient magnetic field coil: (a) to strengthen the strength of thegradient magnetic field coil bobbin; (b) to limit amplitude of thevibration by increasing the weight of the gradient magnetic field coil;(c) to insert a number of lead balls in the structure of the gradientmagnetic field coil to absorb the vibration energy of the coil byconverting it into the frictional heat generated between said leadballs; and (d) to reduce the noise of the gradient magnetic field bygenerating sound having the reverse phase of the noise.

[0009] Each of the above-described methods has achieved about 10 dBnoise reduction. However, in spite of the amount of noise reduction,those methods have some another problems, that is, the weight of thegradient magnetic field coil is increased and the control procedurebecomes complicated when using those methods.

[0010] Concerning said problems, an MRI apparatus employing a magnet ofsolenoidal coil type of which noise reduction effect is improved isproposed in Japanese Patent Laid-open Publication No. JP-A-10-118043. Inthis MRI apparatus, the gradient magnetic field coil is covered withequipment that can vacuumize said coil, and is not put on the magnet butis independently installed on the magnet-installing floor. According tothis method, both the air-propagated vibration resulting from drive ofthe gradient magnetic field coil and the vibration that is propagatedthrough fixed objects into the magnet can be reduced. Consequently,noise of the whole apparatus resulting from the vibration of thegradient magnetic field coil can be reduced by 20-30 dB.

[0011] However, it is difficult to apply this method to the open MRIapparatus: that is, in the open MRI apparatus, the gradient magneticfield coil is divided into the upper part and the lower part, and theyare arranged near the facing plane of the magnet. Therefore, the methodof fixing the upper part of the coil that can secure the openness has tobe devised.

[0012] The MRI apparatus has problems of reducing vibration and noise ofthe gradient magnetic field coil, as well as of reducing eddy currentgenerated on the surface of the magnet vessel placed near the gradientmagnetic field coil when the pulse current is transmitted to said coil.

[0013] In the MRI apparatus, the gradient magnetic field coil is placedvery close to the magnet vessel so as to secure as wide examinationspace as possible. Thus, when the pulse current is applied to saidgradient magnetic field coil, a sudden flux change occurs in the spacesurrounding said magnet vessel with each rise and fall of-the pulse.This flux change causes eddy current to be generated on the surface ofthe magnet vessel placed near the gradient magnetic field coil. Thedirection of said eddy current generated on the surface of the magnetvessel is reverse to that of the gradient magnetic field generated bysaid gradient magnetic field coil, and thus hinder the rising andfalling of the gradient magnetic field. This effect causes variance ofthe amount of the gradient magnetic field which must be applied, andinterferes with execution of the high-speed imaging.

[0014] Therefore, methods to prevent such effect have beenconventionally applied to the apparatus. One of them is to put an activeshield coil on the gradient magnetic field coil. When the active shieldtype gradient magnetic field coil is used in the open MRI apparatus, apair of the gradient magnetic field coils is arranged above and belowthe examination space, and behind them a pair of shield coils is placed.Here, the gradient magnetic field coils have axes respectively in thex-, y-, and z-axes directions so as to generate the gradient magneticfields in these three directions in the examination space.Correspondingly, the shield coil also have axes respectively in the x-,y-, and z-axis directions.

[0015] The relation between the gradient magnetic field coil and theshield coil is described with reference to the figures to show itsimply. FIG. 28 shows a gradient magnetic field coil 610 b in the xdirection, a shield coil 620 b in the x direction, and the magnet vessel21 b. Current in the direction of the arrow M1 (counterclockwise) and ofthe arrow M2 (clockwise) is applied to the gradient magnetic field coil610 b, and current in the direction of the arrow S1 (clockwise) and ofthe arrow S2 (counterclockwise) to the shield coil 620 b. At this point,the magnetic fields in the direction of the arrow U1 (upward) and of thearrow D1 (downward) are generated by the gradient magnetic field coil610 b, and the magnetic fields in the direction of the arrow D2(downward) and U2 (upward) are generated by the shield coil 620 b. Whenthose gradient magnetic fields are combined with the gradient magneticfield generated by the gradient magnetic field coil symmetrically placedacross the examination space, there are generated the upward gradientmagnetic field indicated with the arrow UX in the positive region in thex direction, and the downward gradient magnetic field indicated with thearrow DX in the negative region in the x direction.

[0016]FIG. 29 is a graph showing the calculated magnetic field leakagesof the coils 610 b and 620 b on the observation line 650 shown by abroken line on the magnet vessel 21 b. Referring to FIG. 29, thehorizontal axis represents the distance from the center of the magnetvessel, and the vertical axis represents the strength of the magneticfield leakage. The curved line M indicates the magnetic field leakagedue to the coil 610 b, the curved line S represents the magnetic fieldleakage due to the coil 620 b, and the curbed line T represents thecomposite value of those magnetic field leakages. In the detail of saidcurved line T shown in FIG. 30, the magnetic field leakage is canceledby both of those coils in the region near the magnet vessel. However, itsuddenly becomes large a certain distance away from the center, that is,at the position on the outside of the radius of the gradient magneticfield coil.

[0017] The reason for it may be that the coil conductor of the shieldcoil is arranged over a wider range than the main coil conductor of thegradient magnetic field in order to raise the shielding effect, and themagnetic field strength on the surface of the magnet vessel is inverselyproportional to the square of the distance from the surface of thecryostat of the coil generating the magnetic field. FIG. 31 shows howthe gradient magnetic field coil 610 a and the shield coil 620 a arearranged relative to the surface of the cryostat and the z direction,using a cross-sectional view along the observation line 650. The shieldcoil 620 a and the gradient magnetic field coil 610 a are arranged belowthe cryostat 21 a. For example, six conductors are arranged on theshield coil surface 410 of the shield coil 620 a, and eight conductorsare arranged on the main coil surface 400 of the gradient magnetic fieldcoil 610 a. Said coil conductors of the shield coil 620 a are arrangedmore extensively than said coil conductors of the gradient magneticfield coil 610 a. When the pulse current is applied to these coils, amagnetic field distribution is generated by the coil conductors near theouter circumference of the coils on the surface of the cryostat 21 a, asillustrated in FIG. 32. Referring to FIG. 32(a), 610 a designates thecoil conductors of the gradient magnetic field coil, 630 designatesdistribution of the magnetic field generated by one coil conductor ofthe coil 610 a, and 640 designates the composition of the magnetic fielddistributions generated by a plurality of the coils 610 a. Referring toFIG. 32(b), 620 a designates the coil conductors of the shield coil, 650designates the magnetic field distribution generated by one of theconductors of the coil 620 a, and 660 designates the composition of themagnetic field distributions generated by a plurality of conductors ofthe coil 620 a. FIG. 32(c) shows the difference between these magneticdistributions 640 and 660. The parts shaded with oblique lines are themagnetic fields which are not canceled, and this magnetic flux is leakedto the cryostat 21 a. Consequently, the magnetic field indicated withthe oblique lines leaks to the magnet vessel, and thus eddy current isgenerated on the surface of the cryostat 21 a.

[0018] The present invention is made with regard to the aspectsdescribed above. The first object of the present invention is to securethe air space around the magnet, and to reduce vibration of the gradientmagnetic field coil resulting from driving of gradient magnetic fieldand noise accompanying said vibration in an open MRI apparatus.

[0019] The second object of the present invention is to provide an openMRI apparatus in which the uniformity of the static magnetic field isnot disturbed due to the vibration of the gradient magnetic field coiltransmitted to the static magnetic field generating magnet.

[0020] The third object of the present invention is to provide an openMRI apparatus in which comfortable conditions for examination can beprovided to the object, and the MR-interventional procedure usinghigh-speed imaging can be performed with low noise.

[0021] Further, the fourth object of the present invention is to providean active-shielded gradient magnetic field coil for reducing magneticfield leakage generated at the edge area of the gradient magnetic fieldcoil when pulse current is applied to the gradient magnetic field coil,that is, for reducing eddy current generated on the surface of themagnet vessel, and as well to provide an open MRI apparatus using saidactive-shielded gradient magnetic field coil by which an excellent imagecan be obtained.

SUMMARY OF THE INVENTION

[0022] To achieve the first to third objects described above, an MRIapparatus comprises:

[0023] a pair of static magnetic field generating means;

[0024] a pair of gradient magnetic field generating means for applying agradient of magnetic field strength to the magnetic field generated bysaid static magnetic field generating means; and

[0025] yoke unit for supporting said pair of static magnetic fieldgenerating means,

[0026] wherein each of said pair of gradient magnetic field generatingmeans does not touch said static magnetic field generating means, and isfixed by said yoke unit.

[0027] Further, the MRI apparatus according to the present invention ischaracterized in that said static magnetic field generating meanscomprises air space penetrating in the same direction as the magneticfield direction, and the gradient magnetic field generating means isfixed by said yoke unit through said air space.

[0028] Further, with regard to said gradient magnetic field generatingmeans, its outer circumference may be fixed to the supporting membersurrounding said static magnetic field generating means.

[0029] By directly fixing the gradient magnetic field coil to the yokeunit having large mass, the vibration of the coil is suppressed, andthus the noise caused by this vibration can be reduced. Since the massof the yoke unit is generally considerably larger than the staticmagnetic field generating magnet, the vibration of the gradient magneticfield is not transmitted to the static magnetic field generating magnet.Thus, the vibration propagated through fixed objects can be effectivelysuppressed, and the effect that the vibration has on the static magneticfield generating magnet, that is, fluctuation of the static magneticfield and resultant deterioration of image quality can be, prevented.Further, since the yoke is placed on both sides of the static magneticfield generating magnet, the gradient magnetic field coil can be fixedwithout providing special space on the magnet-installing floor or thesides of the apparatus. Thereby, the MR-interventional procedure can beperformed unhindered by the structure of the apparatus.

[0030] To achieve the fourth object of the present invention, thegradient magnetic field coil comprises a main coil for generating agradient magnetic field on its front side, the conductor of which isspirally wound on a substantially flat plane, and a shield coil forgenerating a magnetic field to cancel at least a part of the magneticfield generated by said main coil mainly at its rear side, theconductors of the shield coil spirally wound on as substantially flatplane at the rear side of said main coil, wherein between the planewhere said main coil is arranged and the plane where said shield coil isarranged, at least one plane where a third-layer coil is arranged isprovided, at least one coil conductor being arranged on this third-layercoil plane.

[0031] Desirably, the coil conductors arranged on the coil planes areconnected to a coil conductor surrounding said shield coil, and currentin the same direction of said shield coil is applied to said coilconductors.

[0032] Further, to achieve the fourth object of the present invention,the gradient magnetic field coil comprises a main coil for generating agradient magnetic field, the conductor of which is spirally wound on asubstantially flat plane, and a shield coil for generating a magneticfield to cancel at least a part of the magnetic field generated by saidmain coil mainly at its rear side, the conductor of which is spirallywound on a substantially flat plane on the rear side of said main coil,wherein current applied to the coil conductors surrounding said shieldcoil may be separated into current in more than two diverging coilconductors arranged on the outside of said coil conductor.

BRIEF DESCRIPTION FOR DRAWINGS

[0033]FIG. 1 shows the whole structure of an MRI apparatus to which thepresent invention is applied.

[0034]FIG. 2 is a cross-sectional view showing the feature of the MRIapparatus in the first embodiment of the present invention.

[0035]FIG. 3 is a perspective view showing a cryostat of asuperconductive magnet for the MRI apparatus shown in FIG. 1.

[0036]FIG. 4 is a perspective view showing the whole superconductivemagnet for the MRI apparatus shown in FIG. 1.

[0037]FIG. 5 shows a pattern of a gradient magnetic field coil in the xdirection.

[0038]FIG. 6 shows a pattern of a gradient magnetic field coil in the zdirection.

[0039]FIG. 7 is a cross-sectional view showing the feature of an MRIapparatus in the second embodiment of the present invention.

[0040]FIG. 8 is a cross-sectional view showing the feature of an MRIapparatus in the third embodiment of the present invention.

[0041]FIG. 9 is a cross-sectional view showing the feature of an MRIapparatus in the fourth embodiment of the present invention.

[0042]FIG. 10 is a top view of FIG. 9.

[0043]FIG. 11 is a top view of the first variation of the embodimentshown in FIG. 9

[0044]FIG. 12 is a top view of the second variation of the embodimentshown in FIG. 9.

[0045]FIG. 13 is a perspective view showing the structure of asupporting member of the gradient magnetic field coil, and disposal of acable and a pipe for cooling in the embodiment shown in FIG. 9.

[0046]FIG. 14 is a perspective view of the outer appearance of an MRIapparatus in the fifth embodiment of the present invention.

[0047]FIG. 15 is a cross-sectional view of a lower magnet according tothe sixth embodiment of the present invention.

[0048]FIG. 16 is a cross-sectional view of an upper magnet according tothe seventh embodiment of the present invention.

[0049]FIG. 17 is a top view of FIG. 16.

[0050]FIG. 18 is a cross-sectional view showing the feature of an MRIapparatus in the eighth embodiment of the present invention.

[0051]FIG. 19 is a perspective view of a supporter of a gradientmagnetic field coil in the tenth embodiment of the present invention.

[0052]FIG. 20 shows one preferable embodiment of manner of supporting agradient magnetic field coil according to the present invention and across-sectional view showing its supporter.

[0053]FIG. 21 is a diagram showing the positional relation between agradient magnetic field coil which is an invention related to thepresent invention and the magnet vessel in the first embodiment.

[0054]FIG. 22 is a distribution map of magnetic field strength in theperiphery of the gradient magnetic field coil shown in FIG. 21.

[0055]FIG. 23 is a diagram showing a pattern of an active shield coil tobe placed on a gradient magnetic field coil in the directionperpendicular to the static magnetic field.

[0056]FIG. 24 is a diagram showing a variation of the pattern of theactive shield coil.

[0057]FIG. 25 is a diagram showing the positional relation between agradient magnetic field coil which is an invention related to thepresent invention and a magnet vessel in the second embodiment.

[0058]FIG. 26 is a distributional map of magnetic field strength in theperiphery of the gradient magnetic field coil shown in FIG. 25.

[0059]FIG. 27 shows the pattern of the shield coil shown in FIG. 25.

[0060]FIG. 28 is an explanatory drawing of the principle of generationof a gradient magnetic field.

[0061]FIG. 29 is a graph representation showing relation betweenmagnetic fields generated by the gradient magnetic field coil and theshield coil and magnetic field leakage.

[0062]FIG. 30 is an enlarged view of FIG. 29 showing the magnetic fieldleakage.

[0063]FIG. 31 is a diagram showing positional relation between aconventional gradient magnetic field coil of active shield and a magnetvessel.

[0064] FIGS. 32(a)-(c) are distribution maps of the magnetic fieldleakage in the periphery of the gradient magnetic field coil shown inFIG. 31.

BEST MODE FOR CARRYING OUT THE INVENTION

[0065] Hereinafter, the support structure of the gradient magnetic fieldcoil in the open MRI apparatus in a preferable embodiment of the presentinvention will be described.

[0066]FIG. 1 shows the whole structure of an MRI apparatus to which thepresent invention is applied. This MRI apparatus comprises a pair ofstatic magnetic field generating magnets 2 above and below the space foraccommodating an object 1 to be examined, gradient magnetic field coils3 placed in the space between said pair of static magnetic fieldgenerating magnets 2, RF coils 5 in the space between said gradientmagnetic field coils 3, and a detecting coil 7 for detecting an NMRsignal generated from the object 1. The gradient magnetic field coils 3and the RF coils 5 are plate-shaped and are arranged at the top andbottom of the space for accommodating an object so as not to hinderaccess to this space.

[0067] This MRI apparatus further comprises a sequencer 9 forcontrolling operation timing for each coil, a computer 10 forcontrolling operation of the apparatus and for processing the NMR signalto make an image, and a table 14 for placing the object 1 in the centerspace of the static magnetic field generating magnet 2.

[0068] The static magnetic field generating magnet 2 comprisessuperconductive magnets separated vertically in the embodiment shown inthe figure. The pair of thus separated superconductive magnets arrangedopposite to each other vertically generates a uniform static magneticfield in the direction perpendicular to the longitudinal direction ofthe object 1. Its magnetic field strength is, for example 1.0 tesla, andthe magnetic flux is directed from the floor to the ceiling, asindicated by an arrow 15. The uniformity of the magnetic field isadjusted to be roughly equal to or below 5 ppm in a spherical volume inthe space in which the object 1 is laid. For the adjustment of themagnetic field uniformity, passive shimming is employed, in which aplurality of fragments of ferromagnetic substances (not shown) areattached on the surface of the superconductive magnet.

[0069] Further, an iron yoke (yoke unit) 16 is provided so as tosurround the upper and lower superconductive magnets. The iron yoke 16comprises a magnetic circuit together with the superconductive magnets,thus lowers density of the magnetic flux that leaks from the magnet.

[0070] The gradient magnetic field coil 3 comprises three pair of coilsperpendicular to one another wound to vary the magnetic flux density inaccordance with the distance in the x, y, and z directions, respectivelyconnected to the gradient power supply source 4 to constitute a gradientmagnetic field generating means. The gradient magnetic fields Gx, Gy,and Gz on the three axes can be applied to the object 1 by driving thegradient magnetic field power source 4 according to a control signalsent from the sequencer 9 to change current value applied to thegradient magnetic field coil 3. Said gradient magnetic fields are usedto set the particular portion of the object to be imaged, and detect thespatial distribution of the NMR signals obtained from the examinedregion.

[0071] Each of the gradient magnetic field coils 3 at the top and bottomof the measurement space is formed by providing the coils in the x, y,and z directions together in one plane and fixing them to the iron yoke16. The structure of the gradient magnetic field coils 3 andinstallation of them to the iron yoke will be described later.

[0072] The RF coils 5 are connected to an RF power amplifier 6 togenerate an high frequency magnetic field for exciting nuclear spins(Proton is usually used.) existing in the examined region of the object1. The RF power amplifier 6 is also controlled by a control signal sentfrom the sequencer 9.

[0073] The detection coil 7 is connected to a receiver 8 to form meansfor detecting the NMR signals. The receiver 8 amplifies and demodulatesthe NMR signals detected by the detection coil 7, and as well convertsthem into digital signals that can be processed by the computer 10. Theoperation timing of the receiver 8 is also controlled by the sequencer9.

[0074] The computer 10 performs image reconstruction using the NMRsignals converted into digital quantity and calculation of spectra etc.,and as well controls operation of each unit in the MRI apparatus with apredetermined timing through the sequencer 9. The computer 10, a memoryunit 11 for storing data, a display 12 for displaying processed data,and an operational console 13 comprise a data processing system.

[0075]FIG. 2 shows in detail the structure of the static magnetic fieldgenerating magnet 2 and the gradient magnetic field coils 36 of the openMRI shown in FIG. 1 in the first embodiment of the present invention.

[0076] First, the structure of the static magnetic field generatingmagnet 2 is described in detail. The static magnetic field generatingmagnet 2 comprises a pair of static magnetic field generating coils,upper and lower, doughnut-shaped cryostats 21, liquid helium tanks 24surrounded by thermal shield plates 22 are placed within the cryostats21, and superconductive coils 23 are put in the liquid helium tanks 24.The inside of the cryostats 21 is evacuated so as to maintain lowtemperature of the liquid helium. The liquid helium tanks 24 and thethermal shield plates 22 are supported by adiathermic wires (not shown)for preventing outside heat from entering so as to reduce evaporation ofthe liquid helium. Incidentally, although only single thermal shieldplates are shown in FIG. 2, usually a plurality of the thermal shieldplates are arranged. Further, although both of the upper and lowermagnets have a single superconductive coil 23 built-in FIG. 2, aplurality of superconductive coils respectively having different sizemay be installed in order to improve uniformity of the static magneticfield, or to reduce magnetic field leakage to the outside.

[0077] Further, these cryostats 21 above and below the measurement spaceare connected to each other by two connecting tubules in order toequalize the amount of liquid helium in each of the liquid helium tank,as shown in FIG. 3. Incidentally, a cryo-cooler 32 is installed in theupper cryostat 21 in order to reduce consumption of the liquid helium.

[0078] About 400-ampere direct current is applied to the superconductivecoils 23, thus a 1.0-tesla magnetic field strength is generated. Theiron yoke 26 comprising the magnetic circuit is placed around the upperand lower cryostats 21 in order to prevent the magnetic flux of such ahigh magnetic field from leaking outside and to confine the volume ofthe high magnetic field to the minimum.

[0079]FIG. 4 shows the static magnetic field generating magnet combinedwith the iron yoke shown in FIG. 3. As shown in the figure, the ironyoke is comprised of an upper plate 41, a lower plate 42, and right andleft column 43 and 44, to form an effective magnetic circuit for themagnetic flux generated by the superconductive coil. The position of theright and left columns 43 and 44 is shifted rearward so as to broadenthe space in front of the static magnetic field generating magnet.

[0080] Generally, it is desirable that magnetic field leakage is keptwithin the examination room (5×8 m, usually) in which the staticmagnetic field generating magnet is provided. Thus, with regard to theiron yoke comprising the magnetic circuit, the product of the saturationmagnetic flux density characteristic of iron times the cross sectionalarea of the yoke must be equal to or more than the value of magneticflux generated by the superconductive coil.

[0081] Next, the pair of gradient magnetic field coils 3 of flat-platetype fixed to said iron yoke is described. FIG. 5 shows a pattern of thex-axis gradient magnetic field coil of flat type. The white regiondesignates conductor, and the black line designates insulation part madeby removing the plate conductor with etching. The points 51 and 52 inthe pattern are connected with a conducting wire 53. Thus, the linelinking the terminals 54 and 55 forms a unicursal pattern.

[0082] When predetermined voltage is applied such that current isapplied in the direction of an arrow 56 between the terminals 54 and 55of the x-axis gradient magnetic field coil, magnetic flux from the backside to the top side of the paper is generated in the right half of thepattern, and that from the top side to the back side in the left half.Thus, a magnetic field gradient in the x-axis can be provided to thestatic magnetic field in the direction perpendicular to the paper (the zdirection).

[0083] The y-axis gradient magnetic field coil is omitted in the figuresince the construction of it is entirely the same as that of the x-axisgradient magnetic field coil. But, it is arranged perpendicular to thex-axis gradient magnetic field coil. Thus, magnetic field gradient inthe y direction can be provided to the static magnetic field by applyinga predetermined voltage to the terminal of the y-axis gradient magneticfield.

[0084] The z-axis gradient magnetic field coil has a single scrollpattern shown in FIG. 6. Two coils having such a pattern are arrangedopposite to each other across the examination space. By applyingpredetermined voltage between terminals 61 and 62 that the currents ofthe tow flow in opposite directions, magnetic flux having strengthgradient in the z direction (perpendicular to the paper) is generated.

[0085] The upper and lower gradient magnetic field coils 3 are formed bystacking the x, y, and z-axis gradient magnetic field coils having theabove-described patterns through insulation material such as epoxyadhesive into one body. The gradient magnetic field coils 3 having suchstructure are fixed to the iron yoke through coil supporting pole 27 andscrews 28 passing through the doughnut-shaped central air space of thestatic magnetic field generating magnet 2, as shown in FIGS. 1 and 2.The outside diameter of the supporting pole 27 is determined to besmaller than the diameter of the air space such that said supportingpole 27 does not touch the cryostats 21 (their inner wall defining theair space). Further, nonmagnetic metal (for example stainless steel,aluminum, etc.) is used in construction of the supporting pole 27 so asto give it to have sufficient strength against stress.

[0086] In the examination using an MRI apparatus, pulsatile current isapplied to these gradient magnetic field coils in the static magneticfield space in which magnetic flux of, for example 1.0 tesla isgenerated in the z direction. Thus, a complex load works on the gradientmagnetic field coils 3. The load thus generated on the gradient magneticfield coils 3 is applied on the iron yoke 26 through the supporting pole27, and not transmitted directly to the upper and lower cryostats 21.The weight of the iron yoke 26 used in the MRI apparatus of passiveshield type is about 35 tons, which is enough to absorb energy generatedin the gradient magnetic field coils 3, thus the vibration is preventedfrom transmitting to the upper and lower cryostats 21 through solidpropagation.

[0087] Further, since the device (the supporter 27) for supporting thegradient magnetic field coils 3 can be arranged within the upper andlower cryostats 21, it does not occupy the space surrounding the staticmagnetic field generating magnet 2, thus the operator of theMR-interventional procedure can efficiently utilize this space.

[0088] Next, the support structure of the gradient magnetic field coilin the second embodiment of the present invention is described.

[0089]FIG. 7 shows an open MRI apparatus in the second embodiment. Allof the structure except the installation structure of the staticmagnetic field generating magnets 2 and the gradient magnetic fieldcoils 3 is the same as that shown in FIG. 2 and is omitted in FIG. 7.

[0090] In the second embodiment, the static magnetic field generatingmagnet 2 is also a superconductive magnet. The structure of it isgenerally the same as that in the first embodiment, in which arecomprised a pair of upper and lower cryostats 71, thermal shield plates72 within said cryostats 71, and the liquid helium tanks 74 containingthe superconductive coils 73. The thermal shield plates 72 hereof may beprovided in plural in the same way as in the first embodiment, and thesuperconductive coils 73 may be also installed in plural. Further, theiron yoke 26 comprising a magnetic circuit is installed around the upperand lower cryostats 71.

[0091] However, in this embodiment, the cryostats are notdoughnut-shaped but cylindrical. The pair of the gradient magnetic fieldcoils 3 is fixed to the iron yoke 26 through supporting rings 75surrounding said upper and lower cylindrical cryostats 72.

[0092] In this embodiment, it is possible to make the gradient magneticfield coils 3 have further rigidity since the outer circumference of itis firmly fixed to the supporting rings 75 by screws 76. Thus, theefficiency of reduction of vibration and noise propagated through fixedobjects can be improved. For the material of the supporting rings 75hereof, nonmagnetic metal (for example stainless steel, aluminum, etc.)can be employed as in the first embodiment. However, if ferromagneticsubstance is used for the supporting ring 75, magnetic shielding effectagainst magnetic flux leaked from the sides of the superconductive coils73 can be obtained. Thus, the static magnetic field generating magnet 2can be more compact than that in the first embodiment. By constructingthe static magnetic field generating magnet 2 in the above-describedmanner, the apparatus can provide a preferable space for the operator ofthe MR-interventional procedure.

[0093]FIG. 8 shows an MRI apparatus in the third embodiment with regardto the support structure of the gradient magnetic field coils. In thisembodiment, the structure of installation of the gradient magnetic fieldcoils is the combination of the first and the second embodiments; thatis, in the static magnetic field generating magnets 2, the cryostats 81are doughnut-shaped as in the first embodiment. The gradient magneticfield coils 3 hereof are fixed to the iron yoke 26 by the supportingpole 82 and the screws 84, utilizing the air space of the doughnutshape, and as well the outer circumference of the gradient magneticfield coils 3 is directly fixed to the iron yoke 26 through thesupporting rings 83. In this embodiment, the gradient magnetic fieldcoils 3 are fixed even more strongly since their outer circumference isfirmly fixed to the iron yoke 26 through the supporting ring 83 andtheir center is fixed to the iron yoke 26 by the supporting pole 82.Thus, the vibrational amplitude of the gradient magnetic field coils 3is controlled to further reduce noise caused by the vibration of thegradient magnetic field coils 3. In this embodiment, also, magneticshielding effect can be obtained and the static magnetic fieldgenerating magnet 2 can be more compact by using a ferromagneticsubstance for the supporting rings 83.

[0094] Further, the gradient magnetic field coils 3 are covered with asound absorption mat 85 in this embodiment. Such an embodiment utilizingthe sound absorption mat 85 is suited for damping higher frequencyvibration modes of the gradient magnetic field coils 3. Thus, damping ofhigher frequency noise can be obtained.

[0095] Next, the support structure of the gradient magnetic field coilsin the MRI apparatus in the fourth embodiment is described withreference to FIGS. 9 and 10. FIG. 9 is a vertical section of the staticmagnetic field generating magnet, and FIG. 10 is its top view. Thisembodiment is different from the above-described three embodiments inthe following points: The gradient magnetic field coil supporter iscomprised of the first gradient magnetic field coil supporter ofcylindrical shape and the second gradient magnetic field coil supporterof plate shape. The center of the gradient magnetic field coil 3 aarranged above the measurement space 25 is joined with the firstcylindrical gradient magnetic field coil supporter 101 a arranged withinthe air space provided in the center of the upper cryostat 21 a, andthus the gradient magnetic field coil 3 a is supported. The upper end ofthe first gradient magnetic field coil supporter 101 a is joined withthe center part of the second gradient magnetic field coil supporter 102a which is above the first gradient magnetic field coil supporter 102 a.The second gradient magnetic field coil supporter 102 a is a long andthin plate as shown in FIG. 10, and it is indirectly supported at itsright and left end by the edge of a connection tube 103. As material forthe first gradient magnetic field coil supporters 101 a and 101 b, andthe second gradient magnetic field coil supporters 102 a and 102 b,nonmagnetic material such as metallic material, for example stainlesssteel and aluminum, or synthetic resin, for example glass-reinforcedepoxy are used.

[0096] In this embodiment, between end faces of the second gradientmagnetic field coil supporter 102 above and the connection tube 103,fixing members 104 comprised of vibration damping material such as hardrubber are inserted. By providing damping effect to said fixing members104, vibration transmission to the magnetic device can be suppressed.Alternately, the second gradient magnetic field coil supporter 102 canbe firmly connected to the connection tube 103 by using metallicmaterial such as stainless steel and aluminum for said fixing members104. Here, it is required that vibration of the gradient magnetic fieldcoil 3 a is damped sufficiently by the time it is transmitted to thefixing member 104, or that the part to which the connection tube 103 isfixedly installed has sufficient rigidity not to be affected by thevibration.

[0097]FIG. 10 is a top view of FIG. 9 showing an example of thestructure of the second gradient magnetic field coil supporter on thetop. As seen in the figure, the second gradient magnetic field coilsupporter 102 a on the top is a plate which is symmetrical to the medialaxis of the apparatus, that is, the z-axis. The width of its center partwhere it is joined with the top end of the first gradient magnetic fieldcoil supporter 101 a is broadened, and is gradually narrowed toward theright and left ends. Both ends are supported by the connection tube 103through the two fixing members 104.

[0098] Referring to FIG. 9, the gradient magnetic field coil 3 b belowthe measurement space 25 is a plate like the upper coil 3 a, joined withthe top end of the first cylindrical gradient magnetic field coilsupporter 101 b placed within a hole 100 formed in the center part ofthe lower cryostat 21 b, thus supported. The bottom of the firstgradient magnetic field coil supporter 101 b is joined with the centerpart of the second gradient magnetic field coil supporter 102 b. Thesecond gradient magnetic field coil supporter 102 b on the bottom isalso a plate, joined with the inner circumference of acryostat-supporting base 105, thus supported. In this instance, it ispreferable to insert vibration-damping material similar to the fixingmembers 104 for preventing vibration transmission between the secondgradient magnetic field coil supporter 102 b and the cryostat-supportingbase 105. Depending on the circumstance, the second gradient magneticfield coil supporter 102 b on the bottom may be installed on the floor106, not fixed to the cryostat-supporting base 105.

[0099]FIGS. 11 and 12 are the top views of the first and secondvariations for the embodiment shown in FIG. 9. In either example, theform of the upper and lower gradient magnetic field coil supporters isdifferent from that in the first embodiment, because the arrangement orthe number of the connection tubes 103 joining the upper and the lowercryostats 3 is different. The variation shown in FIG. 11 is an exampleof an asymmetrical structure in which the two connection tubes areoffset behind the z-axis. In this embodiment, the second gradientmagnetic field coil supporter 102 c on the top is a triangular plate,joined with the upper end of the first gradient magnetic field coilsupporter 101 a at its center part around the z-axis, and joined withthe connection tube 103 through the fixing member 104 at both ends.Further, the second gradient magnetic field coil supporter on the bottomis usually joined to the inner circumference of the cryostat-supportingbase 105 and thus fixed in the same way as in the first embodiment, orput on the floor 106, or joined to the lower edge of the connectiontubes 103. Since the connection tubes 103 are arranged behind the z-axisin this embodiment, the front and sides of the measurement space 25 areopen to the outside and so do not make an object feel constrained.

[0100] The second variation shown in FIG. 12 is suitable for symmetricalstructure in which four connection tubes are arranged symmetrically tothe x-axis (horizontal direction) and y-axis (front-to-rear direction)with their center in the z-axis. In this embodiment, the second gradientmagnetic field coil supporter 102d on the top is roughly a rectangularplate, joined with the top end of the first gradient magnetic field coilsupporter 101 a near the z-axis at its center, and the four corners ofthe rectangle joined with the top ends of the connection tubes 103through the fixing members 104. As in the first embodiment, the secondgradient magnetic field coil supporter on the bottom is joined with theinner circumferential of the cryostat-supporting base 105 and thusfixed, or installed on the floor 106, or joined with the bottom end ofthe connection tube 103. In this embodiment, the sides of the rectangleformed by the four connection tubes in the transverse direction are longand those in the front-to-rear direction are short, thus the measurementspace in the front-to-rear direction is opened widely.

[0101] In the above-described embodiments shown in FIGS. 9-12, thenumber of the connection tubes in the static magnetic field generatingdevice is two or four. However, it is not limited to these and may beany number equal to or more than one. The openness of the apparatus isimproved when the number of the connection tubes is few, while it isdeteriorated when the number of the connection tubes is many. Therefore,two connection tubes are often used in the apparatus employing the openmagnet.

[0102]FIG. 13 shows an example of wiring and piping to the gradientmagnetic field coil in said fourth embodiment. Though FIG. 13 shows theupper gradient magnetic field coil 3 a, the example is also applicableto the lower gradient magnetic field coil 3 b. The cable 110 forapplying current from the power unit of the MRI apparatus for drivingthe gradient magnetic field coil is laid on the surface of the uppersecond gradient magnetic field coil supporter 102, and wired to theupper gradient magnetic field coil 3 a via the central hole 102 h of theupper first gradient magnetic field coil supporter 101 a. Further, acooling pipe 111 is provided along with the cable 110 in order to leadrefrigerant or air for cooling the gradient magnetic field coil 3 a.When a copper pipe and the like are used for the conductor of thegradient magnetic field coil, it is also possible to use the conductorof the wiring as the cooling pipe to unify said wiring and piping. Forsaid piping, nonmagnetic substance is used; for example, there are usedpipes made of metallic substances such as stainless steel or aluminum,and those made of synthetic resins such as Teflon or vinyl polymer.

[0103]FIG. 14 is a perspective view showing the fifth embodiment ofsupport structure of the gradient magnetic field coil used in an MRIapparatus according to the present invention. In this embodiment,independent and static objects which are not part of the magneticstructure are provided near the connection tube 103 in order to supportthe gradient magnetic field coil. Referring to FIG. 14, the upper andlower gradient magnetic field coils 3 a and 3 b are supported by thefirst cylindrical gradient magnetic field coil supporters 101 a and 101b as in the first embodiment. Further, these supporters 3 a and 3 b arejoined to the center portion of the second gradient magnetic field coilsupporters 102 e and 102 f which are horizontally (in the x-axisdirection) long and narrow bars, and thus are supported. The secondgradient magnetic field coil supporters 102 e and 102 f are both joinedat both ends with two stabilizing columns 120 provided outside the upperand lower cryostats 3 to the right and left (along the x-axis) near theconnection tubes 103.

[0104] Referring to FIG. 14, there is one assembled body of the upperand lower cryostats 21 containing the upper and lower superconductivecoils and the two connection tubes 103 for connecting said cryostats,which is supported by four cryostat-supporting bases 121. Further, thereis another assembled body containing the second gradient magnetic fieldcoil supporters 102 e and 102 f on the top and bottom for supporting theupper and lower gradient magnetic field coils 3 a and 3 b and the twosupporting columns 120, which are supported by two stabilizing columnsupporting base 122. As a result, the two assembled bodies arecompletely separated so as not to touch each other. By constructing inthis way, the vibration generated at the gradient magnetic field coils 3a and 3 b is completely prevented from transmitted to the cryostats 21containing the upper and lower superconductive coils for generating thestatic magnetic field, thus disturbance of the uniformity of themagnetic field in the measurement space 4 can be eliminated.

[0105] The stabilizing column 120 is a plate, the width of both ends tobe joined with the second gradient magnetic field coil supporters 102 eand 102 f being wide and the center being narrow. Width of the centerpart of the stabilizing columns 120 is roughly the same as that of theouter circumference of the adjacent connection tube 103. It ispreferable that, when the connection tube 103 and the center part of thecolumns 120 are seen from the center of the measurement space 25, theangle of view taken by the two is roughly the same so that they are seento be roughly superposed. By constructing thus, the stabilizing column120 does not obstruct the openness of the apparatus and conditionsdesired by the object can be preferably maintained. Incidentally,nonmagnetic metallic substance such as stainless steel and aluminum isused for the material of the stabilizing column 120 and the stabilizingcolumn supporter 122.

[0106]FIG. 15 is the lower half of a vertical sectional view showing thesupport structure of the gradient magnetic field coil used in the MRIapparatus in the sixth embodiment according to the present invention. Inthis embodiment, the gradient magnetic field coil 3 b is also supportedby the cryostat 21. Referring to FIG. 15, dampers 125 having preferablehardness are provided between the bottom of the lower gradient magneticfield coil 3 b near the outer circumference and the top surface of thelower cryostat 21 b near the outer circumference. The damper 125 is amaterial made of hard rubber or the like for absorbing the vibration,thus preventing shaking of the gradient magnetic field coil 3 b due tothe vibration. Also, it prevents the vibration of the gradient magneticfield coil 3 b from being transmitted to the cryostat 21 b. The dampers125 may be arranged all along the outer circumference, but it is alsopossible to provide a plurality of the dampers 125 at separate points. Amaterial which absorbs vibration such as hard rubber is most suitablefor the dampers 125. However, nonmagnetic metallic material may be alsoused if the structure of the outer circumference of the vacuum vessel 3is hard enough not to be affected by the vibration.

[0107]FIG. 16 is the upper half of a vertical sectional view showing thesupport structure of the gradient magnetic field coil used in the MRIapparatus in the seventh embodiment according to the present invention,and FIG. 17 is a top view of FIG. 16. This embodiment is preferableexample of support structure when the number of connection tubes 103connecting the upper and lower cryostats is one. Referring to FIG. 16,the upper gradient magnetic field coil 3 a is joined with the lower endof the first cylindrical gradient magnetic field coil supporter 101 aarranged within the central hole 100 of the upper cryostat 21 a. Theupper end of the first gradient magnetic field coil supporter 101 a isjoined with one end of the second oval gradient magnetic field supporter102 g at the periphery of the z-axis. The other end of the secondgradient magnetic field coil supporter 102 g on the top is joined withthe top end of the connection tube 103 through the fixing member 104,the second gradient magnetic field coil supporter 102 g thus beingsupported by the connection tube 103. Although the figure shows thesupport structure of the upper gradient magnetic field coil 3 a, thelower gradient magnetic field coil 3 b is supported in the same manner.Incidentally, the lower gradient magnetic field coil supporter may beeither fixed to the inner circumference of the cryostat-supporting baseor installed on the floor. In this manner, the openness around themeasurement space 25, especially to the front and sides is greatlyimproved by supporting the coils with one connection tube, thus theapparatus can provide the object with a feeling of openness.

[0108] In this embodiment, the gradient magnetic field coil is supportedby one connection tube. However, if enough rigidity can not be obtainedby one connection tube, it is also possible to support the gradientmagnetic field coil with the cryostat 21 a by expanding the outercircumference of the second gradient magnetic field coil supporter 102 gand inserting a damper 126 (made of the same material as the damper 125)between said supporter 102 g and the upper cryostat 21 a. The positionfor inserting the damper 126 is preferably the position opposite to theconnection tube 103, as shown in the figure. By supporting the gradientmagnetic field coil 102 g through the damper 126, the vibration of thegradient magnetic field coil is suppressed and thus vibrationtransmission to the upper cryostat can be prevented. Although thedamper, 126 shown in the figure is rectangular, it can be formed withanother shape such as a cylinder.

[0109]FIG. 18 is a vertical sectional view showing the support structureof the gradient magnetic field coil used in the MRI apparatus in theeighth embodiment according to the present invention. In thisembodiment, the upper and lower gradient magnetic field coils arerespectively supported from the ceiling and floor. The upper gradientmagnetic field coil 3 a is supported by the bottom end of the firstgradient magnetic field coil supporter 101 a, the top end of the firstgradient magnetic field coil supporter 101 a is supported at the centerof the plate-shaped second upper gradient magnetic field coil supporter102 i, and the second upper gradient magnetic field coil supporter 102 iis fixed to the ceiling 107. The upper second gradient magnetic fieldcoil supporter 102 i is fixed to the ceiling 107 by mechanicallyconnecting them using bolts or by adhering them. The lower gradientmagnetic field coil 3 b is supported by the top end of the first lowergradient magnetic field coil supporter 101 b, the bottom end of thefirst lower gradient magnetic field coil supporter 101 b supported atthe center of the plate-shaped second lower gradient magnetic field coilsupporter 102 j, and the second lower gradient magnetic field coilsupporter 102 j is buried under the floor 106 so as to be fixed.

[0110] By fixing the upper and lower gradient magnetic field coilsrespectively on the ceiling and floor in the above-described manner, thesupporting system of the gradient magnetic field coils is separated fromthat of the cryostat covering the static magnetic field generatingsource, thus the vibration of the gradient magnetic field coils is nottransmitted to the cryostats. This embodiment is preferable when theceiling of the examination room is low.

[0111]FIG. 19 is a perspective view showing a variation of the firstgradient magnetic field coil supporter in the fourth to eighthembodiments. This variation is designed to reinforce the first gradientmagnetic field coil supporter, this reinforcing system only being shownin FIG. 19. Referring to FIG. 19, the first gradient magnetic field coilsupporter 101 comprises an outer cylinder 131, an inner cylinder 132,and eight ribs 133. The outer circumference of the outer cylinder 131 ismade to be a size such that it can pass through the central hole 100 ofthe cryostat 21. Further, the inner circumference of the inner cylinder132 is made to be a size such that it can contain the wiring cable 110and the cooling pipe 111. The ribs 133 are rectangular plates, arrangedin radial directions of the outer and inner cylinders 131 and 132, andjoined with outside circumferential surface of the inner cylinder 132and inside circumferential surface of the outer cylinder 131 byadhesion, welding, or screw-fastening. The number of the ribs is notlimited to eight, and it may be more or less than that. The material forthe outer cylinder 131, the inner cylinder 132, and the ribs 133 is thesame as that of the first gradient magnetic field coil supporter 101. Inthis manner, the rigidity of the first gradient magnetic field coilsupporter is raised by reinforcing it with the ribs, thus the rigidityof the whole supporting system of the gradient magnetic field coil canbe raised.

[0112]FIG. 20 shows improved structure of the gradient magnetic fieldcoil. In this figure, the structure of the upper gradient magnetic fieldcoil 3 a and the first upper gradient magnetic field supporter 101 a tobe joined with the coil 3 a is illustrated. The structure of the lowergradient magnetic field coil is the same as that of the upper coil.Referring to FIG. 20, the gradient magnetic field coil 3 a in thisembodiment comprises a main coil 35, a shield coil 36, an interlayer 38,an insulation layer 39, and the fixing plate 37, the whole body forminga flat plate. The main coil 35 and the shield coil 36 are separated byan adequate interval by inserting the interlayer 38, the main coilplaced on the side closer to the center of the measurement space. Theinterlayer 38 isolates and supports the main coil 35 and the shield coil36, and so glass-reinforced epoxy or the like is used for the materialof this interlayer 38. Any nonmagnetic material having electricinsulation and mechanical strength may be used as the material. Betweenthe shield coil 36 and the fixing plate 37 is inserted the insulationlayer 39, and the fixing plate 37 is joined at its center with the lowerend of the first upper gradient magnetic field coil supporter 101 a,thereby being supported. The fixing plate 37 is designed to fix thewhole body of the gradient magnetic field coil 3 a, which is made ofmetallic material such as stainless steel and aluminum or electricinsulating material having mechanical strength such as glass-reinforcedepoxy. The insulation layer 39 is designated to isolate the shield coil36 and the fixing plate 37, which is made of material having electricinsulation such as glass-reinforced epoxy and Bakelite. Respectiveelements of the gradient magnetic field coil 3 a are joined by adhesionor bolt. The fixing plate 37 and the first gradient magnetic field coilsupporter 101 a are joined by adhesion or welding. According to thisembodiment, the gradient magnetic field coil is provided with layershaving mechanical strength between the main coil and the shield coil,and the supporting member is provided at the rear of the shield coil.Thus, the structure of the joint with the gradient magnetic field coilsupporter becomes simple and the rigidity of the whole gradient magneticfield coil can be raised. Incidentally, only the interlayer or only thefixing plate may be provided and the other may be left out, as required.

[0113] In the above-described, embodiment, the superconductive magnet isused as the static magnetic field generating magnet, where high effectcan be expected by using the present invention. However, the presentinvention can be applied not only to the open MRI apparatus using asuperconductive magnet but also those using a permanent magnet or aresistive magnet.

[0114] According to the present invention described above, in an MRIapparatus comprising an open superconductive magnet, a gradient magneticfield coil supporter for supporting the gradient magnetic field coil isextended outside the magnetic device via a central hole provided aroundthe center axis of a vessel containing the static magnet fieldgenerating source, and is supported by objects outside the magneticdevice. In this manner, the gradient magnetic field coil dose notdirectly touch the magnetic device, and thus the transmission of thevibration generated by the gradient magnetic field coil to the staticmagnetic field generating source can be suppressed. As a result,uniformity of the static magnetic field in the measurement space is notdisturbed by the vibration of the gradient magnetic field coil and isstably maintained, and thus a preferable MR image can be obtained.

[0115] Further, according to the present invention, the periphery of themeasurement space is opened and the diameter of the gantry of the wholeapparatus becomes small since the gradient magnetic field coil supporteris not placed at any place around the gradient magnetic field coilexcepting its rear side. Thus, the openness and the operationality ofthe apparatus can be improved.

[0116] As described above, the vibration propagated through fixedobjects from gradient magnetic field coil driving can be dampened byattaching the gradient magnetic field coil to the yoke. Further, noisecaused by vibration due to air-propagation can be damped by attaching asound damping cover to the gradient magnetic field coil. In this manner,even when high-speed imaging is performed in the MRI apparatus, thespace around the magnet can be secured, and as well vibration caused bythe gradient magnetic field coil driving and noise due to it can bereduced. In this manner, comfortable conditions for examination and anopen MRI apparatus by which the MR-interventional procedure can be doneare provided to the object.

[0117] Next, a gradient magnetic field coil which is a related inventionof the above-described invention is described in detail.

[0118]FIG. 21 shows a cross sectional view of the main part of an MRIapparatus mounting a gradient magnetic field coil to which the presentinvention in the first embodiment is applied. In this figure, the crosssection of upper-right half of said part is illustrated with referenceto the measurement space of the MRI apparatus. Referring to FIG. 21, thecenter axis directing to the top and bottom of the apparatus is thez-axis, the direction in which the coil conductors of the gradientmagnetic field coil are arranged is the x-axis direction, and thedirection perpendicular to the diagram is the y-axis direction. In thefigures hereinafter, the coordinate system in FIG. 21 is applied unlessotherwise noted. Although the gradient magnetic field coil according tothis invention is comprised of the three coil elements in the x-, y-,and z-axis directions as described in the section of conventionaltechnology, only one of them will be examined in the descriptionshereinafter. Therefore, the description hereinafter can be applied toany of the x-, y-, and z-axis directions.

[0119] Referring to FIG. 21, the upper part 300 a of a pair of thegradient magnetic field coils 300 of plate type active shield isprovided-below the lower part 210 a of the cryostat 21 a covering theupper part of the static magnetic field generating source. The uppergradient magnetic field coil 300 a comprises a main coil 310 a, a shieldcoil 320 a, and the third-layer coil 330 a. The main coil 310 agenerates a gradient magnetic field in the measurement space at thecenter of the magnet. The shield coil 320 a generates a magnetic fieldfor canceling the magnetic field generated by said main coil 310 a onthe surface 210 a of the cryostat 21 a. Hereinafter, with regard to thedirection of the gradient magnetic field coil 300, the side of the coiltoward the measurement space in which the magnetic field is generated isreferred to as the front side, and the side of the coil toward thecryostat 21 a is referred to as the back side. Although the respectivecoils 310 a, 320 a, and 330 a are comprised of coil conductors and aninsulating holder for holding them, only the arrangement of the coilconductors is shown to simplify the description. The coil conductors aremade of good conductor such as copper wire and copper pipe. Theinsulating holder is made of insulating material having large mechanicalstrength such as glass-reinforced epoxy.

[0120] Here, the third-layer coil 330 a comprises at least one coilconductor (one in the figure) arranged between the surface 400(hereinafter referred to as a main coil surface) on which the coilconductors of the main coil 310 a are arranged and the surface 410(hereinafter referred to as a shield coil surface) on which the coilconductors of the shield coil 320 a are arranged, The position of thesurface 420 (hereinafter referred to as the third coil arrangingsurface) on which the coil conductors of the third-layer coil 330 a ismidway between the main coil surface 400 and the shield coil surface410, but it is not limited to this example and may be closer to the maincoil surface 400 or closer to the shield coil surface 410. Incidentally,the conductor of the third-layer coil 330 a takes up a region of itsplane exceeding in the diameter the maximum diameter of the main coil310 a.

[0121] In this embodiment, current in the same direction as that appliedto the shield coil 320 a is applied to the third-layer coil 330 a. FIG.22 shows the magnetic field strength distribution at this point in oneedge area 220 on the rear side of the gradient magnetic field coil 330a. The third-layer coil 330 a is arranged at a position farther from thesurface 310 a of the cryostat 21 a than the shield coil 320 a.Therefore, the strength distribution of a magnetic field 370 generatedby the sole coil conductor of the shield coil 320 a is high at its peakand its decline from that is rapid. However, the strength of a magneticfield 380 generated by a sole conductor of the third-layer coil 330 a islow at its peak and its decline from that is gentle. By using thethird-layer coil 330 a having such magnetic field strength, thecomposite magnetic field 360 generated by a shield coil made bycombining the third-layer coil 330 a and the shield coil 320 a becomesas shown by the broken line, which is similar in distribution to that ofthe magnetic field 350 generated by the main coil 310 a shown by thesolid line. As a result, the magnetic field 350 generated by the maincoil 310 a can be almost canceled by the shield coil which is madeaccording to this embodiment, thus magnetic field leakage in the edgearea 220 can be reduced.

[0122]FIG. 23 is a perspective view of the right half of said shieldcoil seen from above, showing the positions of the shield coil 320 a andthe third coil 330 a. In FIG. 23, the lines A-A′ and B-B′ areperpendicular lines set on the shield coil surface 410, the line A-A′being a line passing through the z-axis and parallel to the x-axis, andthe line B-B′ being a line passing through the z-axis and parallel tothe y-axis. The cross section of the gradient magnetic coil 300 a inFIG. 21 is the cross section in FIG. 23 taken along the plane includingthe line A-A′ and parallel to the z-axis. In this embodiment, thethird-layer coil 330 a is formed using the idea that a part of theshield coil 320 a can be used as the third-layer coil 330 a when thecurrent in the same direction as that applied to the shield coil 320 ais applied to the third-layer coil 330 a. That is, as shown in FIG. 23,the third-layer coil 330 a is formed by arranging an extension of thecoil conductor of the shield coil 320 a wound on the shield coil surface410 on the third coil arranging surface 420 near the main coil surface400.

[0123] Referring to FIG. 23, the coil conductor of the third-layer coil330 a placed at a distance from the shield coil surface 410 is arrangedalong the incline of a truncated cone of which the upper face is theshield coil surface 410 and the lower face is the third coil arrangingsurface 420. The coil conductor of the third-layer coil 330 a is at theposition farthest from the shield coil surface 410 at the right end (atthe outer circumference of the gradient magnetic field coil 300 a) ofthe cross-section including the line A-A′, that is, at the third coilarranging surface 420, and is the same height as the shield coil surface410 near the line B-B′, and is connected there to the coil conductor ofthe shield coil 320 a.

[0124] Here, the number of the coil conductors of the third-layer coil330 a is not limited to one, and it may be plural. Further, the numberof the third coil arranging surface 420 is not limited to one, and itmay be two or more. Further, in FIG. 23, the current in the samedirection as that applied to the shield coil 320 a is applied to thethird-layer coil 330 a. However, current in the same direction as thatapplied to the main coil 310 a may be applied to the third-layer coil330 a. In this instance, the gradient magnetic field coil 300 a may beformed using the idea of arranging a part of the coil conductors of themain coil 310 a close to the shield coil 320 a. Further, it is alsopossible to make first and second third-layer coils from the main coil310 a and the shield coil 320 a respectively, and arrange them close toeach other. In this manner, by providing two or more third coilarranging surfaces, adjustment of the magnetic field strengthdistribution becomes easier. Thus, by applying this structure to theedge area of the gradient magnetic field coil, magnetic field leakage tothe cryostat in this area can be reduced.

[0125]FIG. 24 shows another example of arrangement of the coilconductors on the third-layer coil. FIG. 24 is a perspective view of theupper part of the shield coil, as was FIG. 23. Referring to FIG. 24, thecoil conductor of the third-layer coil 330 b is connected to the coilconductor of the shield coil 320 a near the line B-B′ on the shield coilsurface 410, laid to the bottom of said truncated cone along the cone'sgradient face, and then wound in arc on the third coil arranging surface420 to the opposite side where the surface ends.

[0126]FIG. 25 shows the second embodiment of the gradient magnetic fieldcoil according to the present invention. In this embodiment, the coilconductor arranged around the shield coil is formed with two divergingcoil conductors to split the coil current. In this manner, value ofcurrent applied to each diverging coil conductor is made small, thus themagnetic field strength distribution formed by each diverging coilconductor in the edge area of the rear side of the gradient magneticfield coil becomes a gentle incline. Referring to FIG. 25, the uppergradient magnetic field coil 500 a comprises a main coil 510 a and ashield coil 520 a. The figure shows the arrangement of the coilconductors only, although the respective coils 510 a and 520 a in thisembodiment are comprised of the coil conductors and the insulatingholder as in the first embodiment. The coil conductors of the main coil510 a are arranged on the main coil surface 400 and those of the shieldcoil are arranged on the shield coil surface 410. The coil conductor ofthe shield coil 520 a is comprised of the coil conductor 530 a of a boldline and the diverging coil conductors 540 a and 550 a of thin lines.The diverging coil conductors 540 a and 550 a are arranged at theoutermost part of the shield coil 520 a near the edge area 220 of therear side of the gradient magnetic field coil 550 a. In the vicinity ofthe edge area 220, the two diverging coil conductors 540 a and 550 a areconnected to the end of the coil conductor 530 a, and the current in thecoil conductor 530 a of the bold line is split into the diverging coilconductors 540 a and 550 a of the thin lines. In this embodiment, thecurrent applied to the outermost coil conductor 530 a of the shield coil520 a is divided in two. However, it is not limited to this example.Current applied to the coil conductors of a plurality of bold lines maybe divided, and the current may be split into more than two divergingcoil conductors.

[0127]FIG. 26 shows the magnetic field strength distribution in the edgearea 220 of the rear side of the gradient magnetic field coil 500 a. Thesplit of current to the diverging coil conductors 540 a and 550 a isperformed such that current is applied more to the inner diverging coilconductor 540 a than to the outer diverging coil conductor 550 a. Bydividing the current to the diverging coil conductors in this manner,the strength distributions of the magnetic fields 590 and 600respectively generated by the diverging coil conductors 540 a and 550 aare formed such that the peak is lower and the incline is gentler incomparison with the strength distribution of the magnetic field 580generated by the coil conductor 530 a. When comparing the magneticfields 590 and 600 generated by the two diverging coil conductors 540 aand 550 a, the peak is lower and the incline is gentler in the strengthdistribution of the magnetic field 600 generated by the outer divergingcoil conductor 550 a, where less current is applied, than in thestrength distribution of the magnetic field 590 generated by thediverging coil conductor 540 a.

[0128] Further, the magnetic field strength distribution in the edgearea 220 generated by the whole shield coil 520 is indicated by thebroken line 570, where the peak is lower and the incline is gentler incomparison with the magnetic field strength distribution generated by aconventional shield coil 620 indicated by the broken line 660 in FIG.32(c). As a result, the magnetic field 570 generated by the whole shieldcoil 520 a has its magnetic field distribution more similar to thatgenerated by the whole main coil 510 indicated by the solid line 560.Thus, the magnetic field leakage of the whole gradient magnetic fieldcoil 500 a to the cryostat in the edge area 220 of the rear side of saidgradient magnetic field coil 500 can be reduced.

[0129]FIG. 27 shows one arrangement of the coil conductors of the shieldcoil 520 a in this embodiment. Referring to FIG. 27, the line connectingthe points C and D indicates the thick coil conductor 530 a, and thelines connecting the points indicate D and E the thin diverging coilconductors 540 a and 550 a. That is, the points C and E are connected toa power source of the gradient magnetic field coil, where the currententering the point C passes through the spiral coil conductor 530 a andis split at the point D into the two diverging coil conductors 540 a and550 a, and the split currents make one revolution and merge at the pointE. The current I applied to the coil conductor 530 a of the shield coil520 a is split at the point D such that current I₁ is applied to theinner diverging coil conductor 540 a of the thin line and current I₂ tothe outer diverging coil conductor 550 a. The value of the split currentis usually determined as I₁>I₂(=I−I₁), as described above. Inconsideration of the respective magnetic field distribution of thediverging coil conductors 540 a and 550 a in the edge area 220, thecurrents I₁ and I₂ are determined such that the magnetic field generatedby the whole shield coil 520 a can cancel the magnetic field generatedby the whole main coil 510 a. When determining values of the splitcurrents I₁ and I₂, their ratio can be controlled by appropriatelyproviding resistance to the diverging coil conductors 540 a and 550 a.Concretely, by appropriately setting the length and cross sectional areaof the diverging coil conductors 540 a and 550 a, resistance of thediverging coil conductors 540 a and 550 a can be changed. Further, it isalso possible to insert a resistor into the diverging coil conductors540 a and 550 a.

[0130] In the description of the embodiments hereof, the first andsecond embodiments are described respectively as independentembodiments. However, it is also possible to combine features of thoseembodiments to carry out. For example, it is possible to divide the coilconductor of the shield coil near the edge area into two diverging coilconductors (that is, to split the current), and arrange one of them onthe shield coil surface and another one on the third coil arrangingsurface between the main coil surface and the shield coil surface; or todivide the coil conductor connected to the shield coil into twodiverging conductors arranged on the third coil arranging surface.

[0131] As described above, in the gradient magnetic field coil accordingto the related invention, the third coil arranging surface is providedto support the coil conductors connected to the main coil or the shieldcoil, at positions between the main coil surface on which the coilconductors of the main coil are arranged and the shield coil surface onwhich the coil conductors of the shield coil are arranged. Thus, in thestrength distribution of the magnetic field generated by the coilconductors arranged on the third coil arranging surface in the edge areaof the surface of the magnet vessel, the height of peak and thegentleness of inclination are between those of the main coil and of theshield coil. As a result, the magnetic field strength distribution ofthe coil conductor arranged on the third coil arranging surface can beused for control of the magnetic field strength distributions of themain coil and the shield coil. Particularly, by connecting to the shieldcoil the coil conductors on the third coil arranging surface, saidmagnetic field strength distribution of the coil conductors on the thirdcoil arranging surface can be used to adjust the magnetic field of theshield coil so as to cancel the magnetic field leakage of the main coil,and thus control of magnetic field cancel becomes easier. In thismanner, magnetic field leakage on the surface of the magnet vessel canbe reduced and generation of eddy current can be suppressed by using inan MRI apparatus the gradient magnetic field coil according to thepresent invention. Thus, negative effect on the static magnetic fieldcaused by said eddy current can be suppressed.

[0132] Further, the coil conductor near the outer edge of the shieldcoil is divided into two or more diverging coil conductors, which arearranged at the outermost winding of the shield coil to split the coilcurrent. Thus, in the strength distribution of a magnetic fieldgenerated by these diverging coil conductors in the edge area on themagnet vessel, the peak becomes lower and the incline becomes gentlerthan those in the magnetic field strength distribution of the coilconductor of the shield coil before it is split. As a result, byapplying to an MRI apparatus the gradient magnetic field coil accordingto the present invention, adjustment of the magnetic field of the shieldcoil for canceling magnetic field leakage in the edge area of the maincoil becomes easier, and thus the magnetic field leakage in the edgearea on the surface of the magnet vessel can be reduced and generationof eddy current can be suppressed. In this manner, adverse influence onthe static magnetic field in the measurement space caused by thegeneration of said eddy current can be suppressed.

1. A magnetic resonance imaging apparatus comprising: a pair of staticmagnetic field generating means; a yoke unit for supporting said pair ofstatic magnetic field generating means; a pair of gradient magneticfield generating means for applying a magnetic field strength gradientto a static magnetic field generated by said static magnetic fieldgenerating means; and supporting means for supporting said pair ofgradient magnetic field generating means, which supporting means fixsaid pair of gradient magnetic field generating means to said yoke unitso as not to touch said static magnetic field generating means.
 2. Amagnetic resonance imaging apparatus comprising: a pair of staticmagnetic field generating means; a yoke unit for supporting said pair ofstatic magnetic field generating means; a pair of gradient magneticfield generating means for applying magnetic field strength gradient toa static magnetic field generated by said static magnetic fieldgenerating means; and supporting means for supporting said pair ofgradient magnetic field generating means, such that there is no solidpropagation of vibration, generated by said pair of gradient magneticfield generating means when they are driven directly, to said staticmagnetic field generating means.
 3. A magnetic resonance imagingapparatus, comprising: a pair of static magnetic field generating means;a yoke unit for supporting said pair of static magnetic field generatingmeans; a pair of gradient magnetic field generating means for applying amagnetic field strength gradient to a static magnetic field generated bysaid static magnetic field generating means; and supporting means forsupporting said pair of gradient magnetic field generating means whichsupporting means fix said pair of gradient magnetic field generatingmeans to said yoke unit so as not to touch said static magnetic fieldgenerating means, wherein said static magnetic field generating meanshas an open hole space penetrating through said static magnetic fieldgenerating means in the same direction as the direction of the magneticfield generated by said static magnetic field generating means, and saidgradient magnetic field generating means is fixed to said yoke unitthrough a support structure body including a supporting member arrangedto pass through said open hole space.
 4. A magnetic resonance imagingapparatus according to claim 1, wherein said gradient magnetic fieldgenerating means is fixed to said yoke unit through a supporting membersurrounding said static magnetic field generating means.
 5. A magneticresonance imaging apparatus according to claim 3, wherein said gradientmagnetic field generating means is fixed at its periphery to asupporting member surrounding said static magnetic field generatingmeans.
 6. A magnetic resonance imaging apparatus according to claim 3,wherein vibration damping material is placed between the periphery ofsaid gradient magnetic field generating means and said static magneticfield generating means.
 7. A magnetic resonance imaging apparatusaccording to claim 3, wherein said gradient magnetic field generatingmeans is covered with a sound absorption mat.
 8. A magnetic resonanceimaging apparatus according to claim 3, wherein said gradient magneticfield generating means is comprised of a first nonmagnetic supportingmember arranged in the open hole space of said static magnetic fieldgenerating means and a second nonmagnetic supporting member arranged atthe rear of said static magnetic field generating means.
 9. A magneticresonance imaging apparatus according to claim 7, wherein said secondsupporting member is fixed on a floor and/or a ceiling.
 10. A magneticresonance imaging apparatus according to claim 3, wherein the supportingmember arranged in the open hole space of said static magnetic fieldgenerating means is in the form of a hollow bar.
 11. A magneticresonance imaging apparatus according to claim 10, wherein a cable forsupplying current to said gradient magnetic field generating means or acooling pipe for sending to said gradient magnetic field generatingmeans refrigerant for absorbing heat generated at said gradient magneticfield generating means is inserted in the open hole space of saidsupporting member.
 12. (Cancel)
 13. A gradient magnetic field coil,comprising: a main coil for generating a gradient magnetic field, whichconsists of a gradient magnetic field coil arranged on a flat plane foruse in an MRI apparatus; a first shield coil arranged at the rear ofsaid main coil, for generating a magnetic field for canceling themagnetic field generated at the rear of said main coil; and a secondshield coil for generating a magnetic field to be combined with themagnetic field generated by said first shield coil, for canceling themagnetic field generated by said main coil, wherein the second shieldcoil takes up a circular region with a diameter greater than the maximumdiameter of said main coil.
 14. A gradient magnetic field coilcomprising; a main coil for generating a gradient magnetic field on itsfront side, an electric conductor of which is spirally wound on analmost flat surface; a first shield coil for canceling at leastpartially the magnetic field generated by said main coil at the rear ofsaid main coil, comprising an electric conductor spirally wound on analmost plain surface; and a second shield coil arranged parallel to thesurface on which said main coil is arranged and the surface on whichsaid shield coil is arranged to generate a magnetic field of suchstrength distribution that, when combined with that of said first shieldcoil, it cancels the magnetic field at the outer edge of said gradientmagnetic field coil.
 15. A gradient magnetic field coil according toclaim 14, wherein said second shield coil has a coil conductor that isconnected to a coil conductor on the periphery of said first shieldcoil, and current in the same direction as that in the coil conductor ofsaid first shield coil is applied to said second shield coil conductor.16. A gradient magnetic field coil according to claim 14, wherein thesurface on which said second shield coil is arranged is set between thesurface on which said main coil is arranged and that on which said firstshield coil is arranged.
 17. A gradient magnetic field coil according toclaim 14, wherein the surface on which the second shield coil isarranged is identical with the surface on which the first shield coil isarranged.
 18. A gradient magnetic field coil comprising: a main coil forgenerating a gradient magnetic field on its front side, comprising anelectric conductor spirally wound on an almost flat surface; a firstshield coil for generating a magnetic field for canceling at leastpartially the magnetic field generated by said main coil in its rear,comprising an electric conductor spirally wound on an almost flatsurface in the rear of said main coil; and a second shield coil,extended to the coil conductor arranged at outermost part of said firstshield coil and connected to said first shield coil, for generating amagnetic field with a strength distribution which, when combined withthe magnetic field generated by said first shield coil, cancels themagnetic field generated by said main coil at its outer edge.
 19. Agradient magnetic field coil according to claim 18, wherein said secondshield coil is provided by dividing said first shield coil conductorinto plural diverging conductors, and the current applied to said firstshield coil is split and thus applied to each of said divergingconductors in a predetermined ratio.
 20. A magnetic resonance imagingapparatus according to claim 1, wherein the plane of the gradientmagnetic field generating means is perpendicular to the direction of thestatic magnetic field.
 21. A magnetic resonance imaging apparatusaccording to claim 2, wherein the plane of the gradient magnetic fieldgenerating means is perpendicular to the direction of the staticmagnetic field.
 22. A magnetic resonance imaging apparatus according toclaim 1, wherein the yoke unit is at least partially made offerromagnetic material.
 23. A magnetic resonance imaging apparatusaccording to claim 1, wherein the supporting means is at least partiallymade of ferromagnetic material.
 24. A magnetic resonance imagingapparatus according to claim 3, wherein the yoke unit is at leastpartially made of ferromagnetic material.
 25. A magnetic resonanceimaging apparatus according to claim 3, wherein the supporting means isat least partially made of ferromagnetic material.
 26. A magneticresonance imaging apparatus according to claim 2, wherein the yoke unithas an energy-absorbing mass to absorb vibrational-energy generated bythe pair of gradient magnetic field generating means, such that there isno solid propagation of vibration to said static magnetic fieldgenerating means.
 27. A magnetic resonance imaging apparatus accordingto claim 2, wherein vibration damping material is placed between aperiphery of said gradient magnetic field generating means and saidstatic magnetic field generating means.
 28. A magnetic resonance imagingapparatus according to claim 2, wherein said gradient magnetic fieldgenerating means is covered with a sound absorption mat.